@Crikeyalmighty You've hit the nail on the head there for me.
My ex husband cheated on me for the first time when I was pregnant with our second child. It was a one-night thing with someone he used to know and he says it was just a kiss (I will never know if that was the truth but even if it was, that's crossing the same line for me).
I was devastated. He was the love of my life and I adored him. Because of that, and being pregnant with an under 2 year old, I stayed and we put it behind us (by never talking about it again, which I regret now).
We had another 10 happy years together and I'm glad I stayed, but if I'm honest, something changed that day I found out and like you say, I knew he was capable of cheating on me. I am sorry to say that attention from a close male (also married) colleague went further than it should have done some years later (we didn't sleep together) and although I knew it was wrong, i didn't feel the guilt I should have done due to what he had already done to me. I would never have done that if he hadn't broken what we had all those years before. The effect on me was far greater than I had realised and I didn't deal with it properly at the time.
I think that's why now my views on a lot of things are different. I still couldn't knowingly stay with a cheater (as hypocritical as that may be) but I also know how things can happen and that having an affair/being unfaithful doesn't = being a bad person or not loving your partner. Before all of this, I would have categorically said as some other have done on here about infidelity. Feelings and behaviours are complicated.
When I found out about his affair it made it easier to end our marriage as it was a full blown, feelings attached affair and I knew I had given our marriage enough chances. A case of sometimes love isn't enough.